Dithering (also known as color quantization or error diffusion) is used in image and video processing. Dithering is often used when an image with high resolution (e.g., 24-bit RGB words having eight bits per color) is converted to image with lower resolution (e.g., 18-bit RGB words with six bits per color). When the conversion is performed by (for example) discarding the lowest 2-bits of a pixel, contouring artifacts often result, especially in smoothly graded regions. Dithering is helpful in reducing contouring artifacts that are often caused by simple quantization techniques.
Different types of dithering can be used to reduce contouring artifacts. Some of the different kinds of dithering include the use of ordered mask values, blue noise mask-based dithering, and error diffusion to neighboring pixels. The dithering can be applied to images from photographs, video sources, and the like.
The process of color quantization involves reducing the number of colors available in a color used in an image. For example, a 24-bit RGB color palette contains around 16.7 million colors. When an 18-bit RGB color palette is generated, the generated color palette contains around 262,000 colors. Dithering techniques appear to reduce the contouring artifacts by taking advantage of certain human visual characteristics. The methods for dithering add high frequency noise to the image such that the noise by itself is not typically visible from distance, but that the average effect of noise addition results in a reduction of the contouring artifacts. At the same time, these techniques work to create the appearance of colors being present from the original color palette in the quantized image.
Floyd and Steinberg dithering is widely used in digital halftoning of intensity images for printing applications. The same concept is also applied for dithering of color images. Dithering of color images is normally used where some conventional display and printing devices do not have the capability to reproduce a true-color (16.7 million colors) image. Such devices often lack the ability to reproduce a true-color image because 6 bit row and column drivers usually cost much less than 8 bit row and column drivers. Accordingly, the last two bits of information are discarded by means of appropriate dithering technique before the truncated image data is sent to the display device. Because color quantization is performed just before the data is sent to row and column drivers of displays, the color quantization is performed at a rate that is suitable for the row and column drivers.
An appreciation of the present invention and its improvements can be obtained by reference to the accompanying drawings, which are briefly summarized below, to the following detailed description of illustrated embodiments of the invention, and to the appended claims.